The objective of this investigation is to study the structural properties of purine and pyrimidine derivatives, and of enzymes that are involved in purine/pyrimidine metabolism. The main emphasis will be on investigations of the crystal structures of nucleosides, nucleotides and purines, and on crystallization and crystallographic studies of enzymes that interact with various purine derivatives. Particular attention will be given to alkaline-earth metal salts of nucleotides; purine analogues that are produced by alkylating agents that act on nucleic acids; and the enzymes adenosine deaminase and purine nucleoside phosphorylase. The crystallographic results will be used to examine the interactions of alkaline-earth metals with nucleotides; the effects that alkylation produces on the geometrical, conformational, tautomeric, hydrogen-bonding, and base-stacking properties of purines; and the structural factors that are involved in interactions of purine derivatives with enzymes. The ultimate aim is to identify specific structural properties that may contribute to the biological effects of purine and pyrimidine analogues.